1. Field of the Invention
The present invention relates to an image reading device for reading an image by moving an image reading unit along a guide shaft.
2. Description of the Related Art
An image reading device (scanner) reads an image of a document and outputs image information to an office machine such as a computer or an image forming apparatus. The image reading device is mounted on an image forming apparatus such as a printer, a fax machine, a copier, or a multifunction machine thereof, or connected to any of those image forming apparatuses as a purchase choice (so-called option).
Reading of an image of a document is performed as follows. An image reading unit is moved along a document while the document is being irradiated with light for reading by a light source placed on the image reading unit (image sensor) or in the vicinity of the image reading unit in which a light-receiving sensor is arranged. At this time, reflected light is guided to the light-receiving sensor by a mirror, and photoelectric conversion is performed in the light-receiving sensor. In this manner, an image of the document is read. The image reading unit is moved by driving of a drive member such as a motor along a guide member such as a guide shaft placed in a predetermined direction.
In the case where the image reading unit moves on the guide shaft, there is a possibility that the moving speed may be irregular due to vibration of the drive member or sliding performance of a sliding portion between the unit and the guide member. In this case, vibration, lateral shaking, chattering, or the like occurs in the image reading unit to degrade quality and resolution of a read image. Therefore, there is a demand for suppressing the occurrence of vibration, lateral shaking, and the like in the drive member and the sliding portion between the unit and the guide member. Further, in recent years, in spite of the fact that image forming apparatus are becoming less expensive, there is an increasing demand for further decreasing vibration, lateral shaking, and the like along with an increase in demand for a high-resolution and high-quality read image.
There has been proposed a structure in which a groove is formed over an entire sliding surface that slides on a guide shaft of an image reading unit to suppress movement to one side and shortage of lubricating grease applied between the sliding surface and the guide shaft, thereby solving the problems of lateral shaking and irregular moving speed of the image reading unit (Japanese Patent Application Laid-Open No. 2006-258968).
There has also been proposed a structure for moving an image reading unit by pulling the image reading unit with a belt, in which a hook portion is provided at a position away from a portion where the belt is fixed to the image reading unit, and the belt is hooked on the hook portion (Japanese Patent Application Laid-Open No. 2010-109399). In the case of the structure described in Japanese Patent Application Laid-Open No. 2010-109399, the belt is hooked on the hook portion, and hence, rotation moment based on tension of the belt occurs. Consequently, a bearing surface of the image reading unit is pressed against a guide rail (guide member), thereby suppressing shaking of the image reading unit during movement.
In the case of the structure described in Japanese Patent Application Laid-Open No. 2006-258968, the vicinity of a top of the groove and the guide shaft are brought into contact at one place in the groove of the sliding surface of the image reading unit. Therefore, in the case where rotation moment acts on the image reading unit during movement of the image reading unit, the contact point between the groove and the guide shaft moves due to the rotation moment. Consequently, the image reading unit is lifted slightly and then falls repeatedly. It is difficult to suppress image blur involved in shaking of the image reading unit which is caused by the repeated lifting and falling of the image reading unit.
In the case of the structure described in Japanese Patent Application Laid-Open No. 2010-109399, it is necessary that the bearing surface be biased to the guide rail by the tension of the belt consistently during movement of the image reading unit. Therefore, it is necessary to provide a guide surface for guiding the belt also on the periphery of the hook portion. Consequently, assembling the image reading unit becomes cumbersome, and production cost rises. Further, a contact area between the bearing surface and the guide rail is large, and hence, when the image reading unit is moved while the bearing surface is biased to the guide rail, a resistance of the movement increases. This makes it necessary to use a high-power motor so as to obtain large torque, which not only raises cost but also increases vibration by the motor, with the result that image blur may not be suppressed easily.